1. Field of the Invention
The present invention relates to a fluorescent water base ink for ink-jet recording which makes it possible to obtain a sufficient optical density value (OD value) and a fluorescence value while securing vivid color development and excellent water resistance.
2. Description of the Related Art
In the ink-jet recording system, ink droplets are formed by using the ink discharge method including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are adhered to a recording objective material such as paper to perform the recording. The ink-jet recording method does not involve the process including, for example, those for the development and the fixation, in which it is easy to realize the color printing. Therefore, in recent years, the ink-jet recording method has remarkably come into widespread use. Recently, the high definition printing and the high speed printing based on the ink-jet printer are rapidly advanced. Further, for example, the color printer, which is capable of performing the printing on regular paper, is dominantly used.
In such circumstances, the performance, which is required for the ink for ink-jet recording to be used for the ink-jet recording method, is to be at a high level. For example, the following performance is required. That is, a recording image, which involves vivid color development, is given. The water resistance is excellent after forming the image. The image is quickly dried when the image is formed on the recording paper, and the rub resistance is satisfactory. No clog-up is caused in the ink flow passage and at the head discharge section of the ink-jet printer, making it possible to perform the stable jetting operation. Neither fading nor discoloration arises after forming the image, and the light resistance and the gas resistance are excellent. In particular, the formation of the recording image having the vivid color development and the water resistance after forming the image are extremely important factors of the performance to realize the image quality equivalent to that of the photograph in which the image is vivid and no blurring occurs.
There have been disclosed inks having vivid color development and excellent water resistance. For example, U.S. Pat. No. 6,176,908 discloses an ink for a postage meter which contains a pigment and a fluorescent dye such as acid dye, direct dye, and basic dye as coloring agents for the ink and which is added with a resin-based dispersing agent for dispersing the pigment. However, in the case of the ink as described above, for example, the acid dye, the direct dye, and the basic dye, which are used as the fluorescent dye, are easily dissolved in water, because the dyes themselves are salts. Even when such a dye is used in combination with the pigment, the water resistance is not sufficient after forming the image.
On the other hand, an ink has been investigate, which is based on the use of a fluorescent dye that is excellent in water resistance as a fluorescent dye having the xanthene skeleton. When such a fluorescent dye is used, it is possible to obtain an fluorescent ink which has extremely excellent water resistance. However, when a fluorescent water base ink for ink-jet recording is manufactured by using a fluorescent dye as described above, a problem has arisen such that any sufficient OD value is not obtained due to the poor water solubility of the fluorescent dye itself.